Use of shape memory alloys in fuel pellet holddown springs

A nuclear fuel rod comprising:

• a zirconium alloy cladding tube;
• zirconium alloy endcaps disposed at each end of said cladding tube;
• a stack of a plurality of axially-extending uranium dioxide fuel pellets axially disposed within said cladding tube;
• and
• a composite holddown spring disposed between at least one of said endcaps and said stack of uranium dioxide fuel pellets, said composite holddown spring further comprising a shape-memory alloy spring portion having an overall transformation temperature range substantially above atmospheric temperature and substantially below an operating temperature experienced by said composite holddown spring, said shape-memory alloy spring portion having a two-way memory characteristic such that said shape-memory alloy spring portion contracts to a first configuration below said overall transition temperature range wherein said axial force exerted by said shape-memory alloy spring portion on said plurality of fuel pellets is minimized and expands to a second configuration above said overall transition temperature range wherein said axial force exerted by said shape-memory alloy spring portion on said plurality of fuel pellets is maximized and
• a conventional metal spring portion having the characteristic that said conventional metal spring portion weakens and contracts at said operating temperature and strengthens and expands at said atmospheric temperature.